The invention relates to lamps, and particularly to LED tubes having one or more LEDs as a light source and enabling a fluorescence tube to be replaced.
Fluorescent lamps are widely used in various environments, such as homes, offices, industry. Fluorescent lamps have longer life, they are more economic and more efficient than incandescent lamps wherein most of the electric power generates heat rather than light. In a conventional fluorescent lamp, the body consists of a straight tube whose length is 15 to 60 times that of the diameter of the tube. The tube may also be bent, in which case the tube may have almost any shape. Fluorescent tubes are low-pressure mercury discharge lamps wherein the inner surface of the tube is coated with a fluorescent material. The structure of a fluorescent tube is very simple; this is illustrated in FIG. 1A. The lamp consists of an airtight glass tube 4 containing a small amount of mercury, an inert gas, a fluorescent coating (luminophore), such as phosphorus, as well as electrodes 2 and 3 (filaments). Both ends of the fluorescent tube are provided with a cap 5 or 6 having two symmetrically placed contact pins 7 and 8 or 9 and 10 to which the electrode 2 or 3 is connected. The current supply to the fluorescent tube is provided via these contact pins 7 and 8, 9 and 10. When the lamp is in operation, the temperature of the electrodes 2 and 3 has to be sufficiently high in order to enable electrons to be released therefrom. At a normal operating voltage, the fluorescent lamp does not ignite without preheating. It is typical of fluorescent tubes (EN 60081) that their cathodes are heated by separate preheat circuits or arrangements. On the other hand, after ignition, a discharging current passing through the tube has to be limited in order for the tube not to become destroyed. Therefore, all fluorescent tubes need a ballast. Conventionally, the ballast has been a ballast-starter combination, which is illustrated in FIG. 1B. When a mains voltage (e.g. 230 VAC) is connected to the lamp, the resistance through the tube is very high, and electric current passes via a ballast L, the electrode 3, a closed starter 11, and the electrode 2. When the electric current passes through the electrodes 2 and 3, it heats the electrodes, causing them to emit electrons which ionize the gas inside the tube. The ionized gas forms a current path through the tube. The current passing through the ballast L generates a magnetic field therein. When, after a while, the starter 11 opens, the magnetic field of the ballast L generates a high voltage between the electrodes 2 and 3, which ignites the lamp.
Today, also electronic ballasts are used. An electronic ballast is also responsible for igniting the lamp, so no separate starter is necessary. The preheating arrangement is implemented either by separate preheating windings or a starter capacitor. This is illustrated in FIG. 1C. An electronic ballast 12 connected to the mains voltage (e.g. 230 VAC) provides a continuous electric current through each electrode 2 and 3. These electric currents are configured so that a voltage difference is formed between the electrodes 2 and 3. When the mains voltage is connected to the ballast 12, the electric current passing through the electrodes heats them quickly, and the emitted electrons ionize the gas contained in the tube. After the gas has become ionized, the voltage difference between the electrodes starts a gas discharge.
A common aim is to replace fluorescent tubes by LED tubes having the same length and values. Their physical dimensions are the same as those of straight fluorescent tubes (e.g. T8 having a 26 mm diameter and being e.g. 60 or 120 cm in length), enabling a LED tube to directly replace a fluorescent tube in an existing fluorescent lamp. Examples of such LED tube lamps are set forth in EP1852648, US2007/0183156, US2010/0002439, and WO2009/131340. The goal is to achieve a long life for the light source and a better luminous efficiency (quantity of light/electric power). In practice, the aim is only to replace a fluorescent tube by a LED tube without changing the structures of the lamp. Some LED tubes work directly with a fluorescent tube ballast, in which case only the starter is to be removed from use. In such a case, the LED tube replacement may take place easily and without any professional assistance.
This causes some problems, the most important one being the danger of an electric shock while installing the LED tube. FIG. 2 illustrates in a simplified manner a fluorescent tube lighting fixture 20 comprising a body 24 containing the necessary electronic structures, such as a ballast 12 and a starter 11, which is usually necessary only in connection with a ballast. The ends of the lighting fixture are provided with tube holders 21 and 22 provided with contact sockets 23 into which contact pins of the ends 26 and 27 of the tube 25 are inserted in order to achieve a mechanical and electrical connection. According to safety regulations in the field, lighting fixtures are to be built such that it is impossible for a user to come into contact with any parts at the mains voltage while replacing a fluorescent tube even if the lamp were connected to the mains voltage. This requirement is met even if the fluorescent tube were replaced such that only one end 27 of the tube 25 is in contact with the contacts 23 of the tube holder 22 while the other end 26 of the tube may freely come into contact with a person carrying out the replacement. This requirement is met since no current flows through the gas-filled fluorescent tube until the gas in the tube is ionized by a starting pulse. Thus, the gas per se in the fluorescent tube serves as an insulating material. The electrical structure of the lighting fixture, again, is such that the generation of a starting pulse requires that both ends of the tube be connected to the contacts of the tube holder. This enables the risk of an electric shock in connection with a fluorescent tube to be prevented while replacing the tube.
In connection with LED tubes, this electrical safety condition is not met. Inside the LED tubes there is provided a printed circuit board or a corresponding structure where the LEDs and their necessary electronic current supply components are installed. The purpose of these components is to convert the supply network alternating voltage into direct voltage and regulate the direct current required by the LEDs. In practice, current flows through these components as soon as voltage is applied thereto, in other words the LED tube is in a conductive state without any separate starting pulse. Therefore, in a use situation, when the LED tube 25 is being installed in the fluorescent tube lighting fixture 20, the contact pins 27 of one end of the LED tube 25 may come into contact with the contacts 23 of the tube holder 22 and the other end 26 of the tube be left outside the lighting fixture such that the person installing or replacing the tube may come into contact therewith, putting him or her at the risk of an electric shock.